1. Field of the Invention
The present invention relates to a tandem type color image forming apparatus that includes a laser driving device and a photoconductive member per each color.
2. Discussion of the Background Art
A conventional tandem type color image forming apparatus generally includes a plurality of image formation units arranged along a transfer belt that conveys a recording sheet so as to form and transfer a plurality of superimposing monochrome images at different image formation times.
Due to employment of the plurality of image formation units, a color image tends to deviate in such a tandem type color image forming apparatus.
Japanese Patent Application Laid Open No. 2000-28459 discusses a technology to reduce such color deviation even when phases of photoconductive drums change. Specifically, a plurality of registration marks having prescribed shapes are formed on a transfer belt by respective image formation units to be detected by an optical sensor. A positional deviation between the registration marks is calculated and updated based on the detection. A position to start writing the image (i.e., registration) is corrected in each color formation in accordance with positional deviation.
Further, Japanese Patent Application Laid Open No. 10-148992 discusses a technology capable of canceling color deviation caused by unevenness of a driving mechanism, such as belt traveling, photoconductive drum rotation, etc., by calculating an amount of correction for each color. Specifically, when the above-mentioned color correction technology is used and color deviation still remains, a plurality of registration patterns are repeatedly formed for each color over one cycle on the transfer belt and are detected by a photoelectric sensor.
Then, a color deviation is calculated over one cycle of the transfer belt. A representing value is obtained by averaging all of the color deviation amounts. A difference between the representing value and the color deviation amount is obtained and stored in a memory as a differential data with it being associated with the corresponding phase. When registration is practically performed, a small number of registration patterns are formed in a short interval between recording sheets on the transfer belt. Then, the registration patterns are detected and color deviation amounts are obtained. The color deviation amounts are corrected with reference to the differential amount of the same phase.
Since only a BK photoconductive member is used in a monochrome image formation mode of such a tandem type color image forming apparatus, phases of color photoconductive members generally need to be the same to the phase of the BK photoconductive member, when the monochrome image formation mode is changed to a color image formation mode.